The present invention relates to a mechanism for cancelling the effects of frictional stress on position detector elements. The invention is particularly useful in liquid level detectors.
In one type of liquid level detector, a float rides on the liquid surface, and a rod or arm structure extends from the float to operatively connect with a readout mechanism--e.g., the slider of a potentiometer or the needle on a gage. Frictional resistances in the system tend to prevent the float from reaching a true equilibrium position relative to the liquid level surface. Eventually, the frictional resistances are overcome, whereupon the float moves toward and often beyond the true equilibrium position. At times the float may be slightly higher than a true equilibrium position, and at times the float may be slightly lower than the true equilibrium position.
The present invention utilizes a disturbance mechanism for periodically jostling the float or an associated motion member, whereby frictional stresses or motion-retarding stresses are overcome. A cyclically-excitable electric sensing means, typically a linear variable displacement transformer, is used as a readout device, whereby the float position is repetitively sensed at spaced time intervals--e.g., ten thousand sensor signals per second. A sampling circuit and an averaging circuit are used in conjunction with the electric sensing means so that the equilibrium position of the float is established by averaging a large multiplicity of output signals generated by the electric sensing means. With the contemplated arrangement, individual signals may be higher or lower than a true equilibrium signal, but the signal average will be representative of the true equilibrium float position, within a comparatively small inaccuracy range.
The disturbance mechanism utilized to jostle the float, or its motion member, may be designed to have only intermittent engagement with the float mechanism, whereby the float is free-floating on the liquid surface during the major portion of each disturbance cycle. The float is then under the control of the disturbance mechanism for only a minor portion of each disturbance cycle. In a typical situation, the disturbance mechanism might be arranged to strike or jostle a float motion member once each second. During each one second period the disturbance mechanism might be in contact with the float motion member less than 100 milliseconds. The aim is to impart a disturbing force to the float motion member to enable the float mechanism to overcome frictional stresses, while at the same time permitting the float motion member to be essentially unrestrained by the disturbance mechanism during the major part of each disturbance cycle.
This invention may be utilized for the detection of leaks in liquid-containment tanks by sensing slight changes in liquid level. The invention has potential application in any detection system wherein the detector element has difficulty in reaching an equilibrium condition due to hysteresis effects.